Conventionally, a liquid photosensitive resin composition is used as an optical waveguide core layer material for an optical/electrical transmission hybrid flexible printed wiring board. Where a pattern of a core layer is to be formed by using the liquid photosensitive resin composition, the core layer is irradiated with ultraviolet (UV) radiation via a photo mask to be thereby formed into a desired core pattern. The photosensitive resin composition has a higher photo-curing sensitivity. However, a coating film of the photosensitive resin composition is liable to be broken because of its surface tackiness when being brought into contact with a roll in a continuous process such as a roll-to-roll (R-to-R) process employed for mass production. Therefore, the photosensitive resin composition is not adaptable to the R-to-R process, resulting in poorer productivity (PTL1).
In order to adapt the photosensitive resin composition to the continuous process such as the R-to-R process, a polymer material which is solid at an ordinary temperature is typically used as a photosensitive resin for the photosensitive resin composition. As the molecular weight of the polymer material is increased, the flexibility of an uncured amorphous film of the photosensitive resin composition is increased, but the patterning resolution (curing sensitivity) is reduced. As the molecular weight of the polymer material is reduced, on the other hand, the patterning resolution is increased, but the flexibility is reduced. In general, there is problematically a trade-off relationship between the flexibility and the patterning resolution of the film. Hence, there is a demand for an optical waveguide core layer material which satisfies the requirements for the flexibility and the patterning resolution of the photosensitive curable film, and a variety of such optical waveguide core layer materials have been proposed (PTL2).
The optical waveguide core layer material should satisfy requirements for various properties such as higher refractive index, higher transparency, higher resolution patternability and higher heat resistance when being cured according to its use purpose. Therefore, considerations are given to selection and balance of ingredients to be blended to satisfy the property requirements.
In order to adapt the optical waveguide core layer material to the R-to-R process for the mass production, as described above, an uncured film of the optical waveguide core layer material is generally provided in the form of a dry film. In the development of the material, the requirements for lower tackiness and higher flexibility of the uncured dry film material for the process adaptability reduce the design flexibility. In addition, it is necessary to provide laminate bases on opposite surfaces of the uncured film to provide the dry film. This is disadvantageous for resource saving and cost saving. Therefore, the adaptability to a wet process is also an important consideration in the development of the material (PTL3).
In view of the foregoing, a photosensitive resin composition is proposed, which is prepared, for example, by blending plural types of resins in addition to a base material of a specific novolak polyfunctional epoxy resin to satisfy the aforementioned property requirements (PTL4).